In a time-of-flight mass spectrometer (TOFMS), various ions derived from a sample are ejected from an ion ejector, and the time of flight required for each ion to fly a certain flight distance is measured. Each ion flies at a speed according to its mass-to-charge ratio m/z. Accordingly, the above-mentioned time of flight corresponds to the mass-to-charge ratio of the ion, and the mass-to-charge ratio of the ion can be determined based on its time of flight.
FIG. 13 is a schematic configuration diagram of a typical orthogonal acceleration TOFMS (hereinafter, it may be referred to as “OA-TOFMS”).
In FIG. 13, ions generated from a sample in an ion source (not shown) are introduced into an ion ejector 1 in the Z-axis direction, as shown by an arrow in FIG. 13. The ion ejector 1 includes a plate-shaped push-out electrode 11 and a grid-shaped extraction electrode 12, which are arranged to face each other. Based on control signals from a controller 6, an acceleration voltage generator 7 applies a predetermined level of high-voltage pulse to either the push-out electrode 11 or the extraction electrode 12, or to both, at a predetermined timing. By this operation, ions passing through the space between the push-out electrode 11 and the extraction electrode 12 are given acceleration energy in the X-axis direction and ejected from the ion ejector 1 into a flight space 2. The ions fly through the flight space 2 which has no electric field, and then enter a reflector 3.
The reflector 3 includes a plurality of annular reflection electrodes 31 and a back plate 32. A predetermined direct-current voltage is applied to each of the reflection electrodes 31 and the back plate 32 from a reflection voltage generator 8. A reflective electric field is thereby formed within the space surrounded by the reflection electrodes 31. The ions are reflected by this electric field, and once more fly through the flight space 2, to eventually reach a detector 4. The detector 4 generates ion-intensity signals according to the amount of ions that have reached the detector 4, and sends those signals to a data processor 5. The data processor 5 creates a time-of-flight spectrum that shows the relationship between the time of flight and the ion-intensity signal, with the point in time of the ejection of the ions from the ion ejector 1 defined as the time-of-flight value of zero, and converts the time of flight to a mass-to-charge ratio based on prepared mass calibration information, so as to create a mass spectrum.
When ions are to be ejected from the ion ejector 1 of the above-mentioned GA-TOFMS, a high-voltage pulse having the magnitude on the order of kV with a short duration needs to be applied to the push-out electrode 11 and the extraction electrode 12. For generating such a high-voltage pulse, a power supply device as disclosed in Patent Literature 1 (it is referred to as a “pulsar power source” in this document) has been conventionally used.
The power supply device includes: a pulse generator for generating a low-voltage pulse signal for controlling the timing of the generation of the high-voltage pulse; a pulse transformer for transmitting the pulse signal from a control-system circuit to a power-system circuit while electrically insulating the control circuit that operates with a low voltage from the power circuit that operates with a high voltage; a driving circuit connected to the secondary winding of the pulse transformer; a high-voltage circuit for generating a high direct-current voltage; and a switching element employing metal-oxide-semiconductor field-effect transistors (MOSFETs) to generate a voltage pulse by turning on and off the direct-current voltage generated by the high-voltage circuit according to a control voltage provided through the driving circuit. Such circuits are not limited to TOFMSs; they are commonly used for generating high-voltage pulses (see Patent Literature 2 and others).
As described above, the TOFMS measures the time of flight for each of the ions, with the point in time of the ejection of the ions or the acceleration of the ions defined as the time-of-flight value of zero. Accordingly, in order to enhance the accuracy in the measurement of the mass-to-charge ratio, the point in time of the initiation of the time-of-flight measurement needs to coincide with the timing of the actual application of the high-voltage pulse to the push-out electrode or the like as much as possible.
The above-mentioned power supply device employs semiconductor components such as complementary metal-oxide semiconductor (CMOS) logic ICs and the MOSFETs, and the pulse transformer, so as to generate the high-voltage pulse based on the low-voltage pulse signal. With these components and elements, a transmission delay occurs between a point in time at which a certain signal is inputted and a point in time at which another signal is output in response to the signal. In addition, a certain degree of time for rise or fall of a voltage waveform (or current waveform) is required for a change in the voltage waveform (or current waveform). Such transmission delay time, rising time, and falling time are not always constant, and change according to the temperature of the components and the elements. Thus, a change in the ambient temperature of the power supply device causes a time discrepancy in the timing of the application of the high-voltage pulse to the push-out electrode or the like, and this time discrepancy causes a mass discrepancy in the mass spectrum to a certain extent.
In order to cope with the problems, a TOFMS disclosed in Patent Literature 3 measures the temperature of an electric circuit, and corrects the measured time-of-flight data according to the temperature measured, so as to resolve a mass discrepancy. In other words, when the ambient temperature of the power supply device differs from, for example, a standard temperature, this method allows an occurrence of discrepancy in the time of flight, and resolves the discrepancy by data processing. In this method, highly accurate correction information that indicates the relationship between the temperature discrepancy and the time-of-flight discrepancy needs to be prepared, so as to correct the time-of-flight discrepancy at high accuracy. However, the time of flight generally varies depending on various factors, for example, not only a temperature in each section, but also installation accuracy of components, such as a reflector and a detector, variation in reflective electric field caused by contamination of a reflector, and the like. Therefore, even when the above-mentioned correction information is prepared on certain conditions, highly accurate correction cannot always be achieved by utilizing the correction information.
Further, the data correction processing made after the measurement takes time and causes as much delay in preparing the mass spectrum. For example, when a mass spectrum obtained from a normal mass analysis should be analyzed in real time to determine a precursor ion for a subsequent operation, i.e., a mass spectrometry/mass spectrometry (MS/MS) analysis, a delay in the MS/MS analysis can occur.